The present invention relates to a catalytic combustor for gas turbines which aims at achieving low NOx combustion and, more particularly, to a catalytic combustor designed to enable fuel to be completely burned at low NOx emission in the entire range from the starting to the rated speed of the turbine.
As compared with the conventional gas-phase combustion system, the catalytic combustion can considerably reduce the NOx emission, can also reduce carbon monoxide and unburnt hydrocarbon, and can raise a combustion amount without increase in pressure loss at the combustor.
In the general catalytic combustion, the reaction rate of fuel in a low gas temperature range is determined by an inherent chemical reaction occurring at the catalyst surface, that is, is in a range of a reaction rate-determination where the mass transfer or heat transfer between the catalyst layer and the gas flow is made faster than the chemical reaction speed. For this reason, the temperature distribution and concentration distribution at the catalyst reaction surface become essentially equal to the temperature distribution or concentration distribution of the gas flow.
As the temperature range, in which the chemical reaction is rate-determined, is exceeded, a region is reached where the chemical reaction speed inherent to substance becomes substantially equal to its maximum speed. As this temperature is reached, transfer of the substance and heat is initiated to occur between the catalyst surface and the gas flow. In this state, the catalyst surface temperature is elevated to a level higher than the gas temperature and, accordingly, the fuel concentration in the vicinity of the catalyst surface is reduced to a level lower than that of the main flow.
As the temperature is further elevated, a region is reached where the reaction speed becomes fast abruptly in proportion to the rate of active substances diffused to the catalyst surface. In this region, since the active substances react immediately after they reach the catalyst surface, the active substance concentration becomes substantially equal to zero. That is, a diffusion rate-determining region is reached where it is the ruling or dominant condition how the active substances reach the catalyst surface. In the diffusion rate-determining region, the diffusion coefficient which the substances have is important. However, since the diffusion coefficient is not so much influenced by the temperature, the reaction speed is brought to a substantially constant level over the broad temperature range.
As the temperature further rises, the reaction speed rises abruptly and, finally, the gas-phase reaction is reached.
As will be understood from the foregoing description, it is advantageous to carry out the catalytic combustion at the level equal to or above the temperature at which the diffusion rate-determining region is reached. This is necessary in practical use.
On the other hand, when the reactivity under the above-described condition is considered, it is needless to say that necessary is the catalyst surface sufficient to receive the active substances diffused, in addition to the temperature condition under which the diffusion rate-determining region is reached. In the actual combustor, however, it is desirable that the combustor body is small in size, and it is not desirable to increase the amount of catalyst in order to obtain sufficient catalyst surface. Effective measures for reducing the overall device dimension are to combine the temperature range in which the diffusion rate-determining region is reached, and the higher temperature range with each other to design a combustor.
Moreover, the relationship between the fuel concentration and the catalytic reactivity is such that the reactivity rises if the fuel concentration is high. The reason for this is that higher the fuel concentration, the higher the heat generation temperature at the catalyst surface, to thereby elevate the gas temperature in the vicinity of the catalyst layer so that temperature reaches a region beyond the temperature range of the diffusion rate-determining stage, i.e., reaches a level at which the uniform gas-phase reaction proceeds. That is, a combustible range, when the actual catalytic combustor is supposed, is limited by the combustion efficiency on the fuel lean side, and is limited by the heat resistant temperature of the catalyst on the fuel too-rich side. Accordingly, the fuel concentration range satisfying both of them is extremely narrowed.
The relationship between the fuel concentration and the turbine load in the general gas turbine for generator is such that the fuel concentration is in a range of from 1% to 2% in the course of the starting of the turbine, and in a range of from 1% to 4% under the load condition. Thus, it is a great problem to achieve complete combustion by the use of catalyst in the region where the fuel concentration varies considerably.
In the prior art published, however, as disclosed in Japanese Patent Laid-Open Application No. 58-92729, emphasis of the consideration about change in fuel concentration is placed on the fuel too-rich side, i.e., on the catalyst heat resistant temperature, and no particular description is made to the combustion performance on the fuel lean side.
As represented by the aforesaid Japanese Patent Laid-Open Application No. 58-92729, an attempt is made in the prior art to use the combustor also when the fuel concentration varies, by arranging a plurality of catalysts different in heat resistant temperature from each other. However, no remarkable consideration is made on such important point that the individual catalysts have their respective inherent lower limits of completely combustible fuel concentration, and it is unavoidable for any catalysts that combustion is effected incompletely if the concentration is out of the above limits, so that the requisite gas temperature is not obtained. That is to say, the catalysts have their respective inherent lower limits of completely combustible fuel concentration, and in case of a combustor such as one for a gas turbine which is used in a broad range of fuel concentration, a problem is how a system is arranged to enable complete combustion in the entire range of the turbine load.